My Xampp on linux is to establish such as this

ServerRoot "/opt/lampp"
DocumentRoot "/home/webadmin/webroot"
<Directory />
    Options FollowSymLinks
    AllowOverride None
</Directory>
<Directory "/home/webadmin/webroot">
    Options Indexes FollowSymLinks ExecCGI Includes
    AllowOverride All
    Order allow,deny
    Allow from all
</Directory>
<IfModule dir_module>
    DirectoryIndex index.html index.html.var index.php index.php3 index.php4
</IfModule>
...

This can lead to a 403. However, after i change DocumentRoot to "/opt/lampp/htdocs", it really works.

Already double checked that /home/webadmin/webroot is readable. What is the problem here?

ServerRoot should be a directory over the DocumentRoot, therefore if you will need to have document root at "/home/webadmin/webroot" you have to set your ServerRoot to "/home/webadmin/webroot" a minimum of... or something like that like "/home/webadmin"